In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
As a replacement to traditional wood products and/or components, plastics and fiber-reinforced plastics have become increasingly dominant as the replacement materials of choice. The reasons for their broad appeal range from their weather resistant qualities, to ease and diversity of manufacturing. Generally speaking, plastics used in the production of construction materials can be grouped into one of two categories. These are thermoplastic plastics (also called thermoplastics) and thermoset plastics (also called thermosets). Thermoplastics dominate the current industry.
The defining characteristic of thermoplastics is that when subjected to heat, they become soft, or plastic. They can be reheated and reshaped many times, or mixed with reinforcing materials prior to cooling as a finished product. This provides advantages for manufacturing processes. However, thermoplastics possess poor structural capabilities. This deficiency has been improved by combining cellulose or other fibers into a molten matrix of common thermoplastics to create composite materials known as fiber reinforced plastics (FRP's) or more specifically fiber reinforced thermoplastics (FRPp's). The resulting FRPp composite material, such as TREX™, although improved, remains limited for use in high performance or other structural applications due to the weak linear bonds of a thermoplastic matrix.
Thermoset plastics are permanent or permanently set when cured. These plastics begin as low viscosity resins that, when mixed with a cross-linking curing agent or other means, become permanently solid. They can be formulated to possess a wide range of properties pertaining to resilience, rigidity, weather resistance (weather-ability), and thermal dimensional stability among others. Most often these resins are combined with reinforcing fibers, such as glass or carbon. The addition of fiber reinforcement to thermosetting plastics greatly improves tensile properties and thermal characteristics. The resin matrix impregnates these high strength fibers and distributes applied stresses to them.
Most applications of fiber reinforced thermoset plastics (FRPs's) focus on developing lighter, stronger, and more durable products. FRPs technology has created very strong and versatile materials. Although having superior performance properties, FRPs technology has not been incorporated into the residential/light construction building industries. In the current market, few, if any, manufacturers consider combining these high tech, high performance composites with inferior materials such as wood or thermoplastics. Although fiber reinforced thermoset plastics, such as fiberglass, provide extremely desirable performance qualities, they lack the look and feel of traditional construction materials, such as wood. Further, once cured, they are difficult to shape, modify or otherwise work with (work-ability) in the field, as is necessary in many conventional uses, such as constructing millwork or detailed areas of buildings.
Several techniques of creating and/or applying FRPs have been implemented in the fabrication of building materials and products. All of the following methods use the fiber-reinforced plastics as a laminated veneer and/or as a visible or exposed component of the final structure. Both the aesthetic qualities of a “plastic” appearance and lack of its ability to be modified after cure limit the considerations of using this type material for many products and end uses. One method laminates a single or several layers of resin impregnated fiber fabric (known as lay-up) over an existing mold, wood frame, or other structure. After curing, the fabric can remain attached as a reinforcing and/or weather resistant veneer, or can be removed using a release agent. Another method uses a slurry-like mix of short fibers and thermoset resin that is dispensed over forms or molds, using a compressed air gun nozzle or “chop gun”. The mix can be dispensed to a varying thickness, and cured using a time dependant, heat sensitive or radiation sensitive catalyst. Once cured, they also may be removed from the form or mold making using a release agent. A more current application that combines saturation and curing is called pultrusion. Pultrusion is the pulling of continuous fibers through a resin bath, and then immediately passing these saturated fibers through a heated die that initiates a heat sensitive curing process. The die, ranges in geometry from basic to elaborate profiles. One application of pultrusion produces flat strips. The cured strips then have one surface roughened or otherwise abraded in order to enable adhesives to bond adequately. This is necessary due to the smooth, rigid surface conditions of the encapsulated fibers of the cured strip. They are then laminated, as a means of reinforcement, to exterior portions of the beam that experience high stresses using known adhesives, such as resorcinol (see, for example, U.S. Pat. Nos. 5,362,545 and 5,885,685).
It is commonly understood certain difficulties occur when combining rigid, planar materials by means of adhesive attachment. The less refined the surface, the more difficult it is to achieve adequate contact, thus a strong and consistent bond. On the other hand, the more surface area in contact, the better the bond. However, a higher degree of refinement, e.g., preparation of the surface, is often associated with increases in time, costs, and other resources. Also, unless the surfaces being bonded are refined to be perfectly uniform, they only come into contact along protruding portions. Therefore, the remainder of the surface area remains either a void, or when possible, filled with excess adhesive.
Several methods exist to add reinforcement to an element. One typical approach is to identify a reinforcing material that exhibits the desirable performance characteristics and laminate it, using high strength adhesives, between layers of the substrate material being reinforced. The most limiting factor is to select a reinforcing material that will not adversely affect the ability to cut or otherwise modify the end product using common tools and methods. One such case uses a strip of high strength aluminum to reinforce a beam made of many laminations of wood strips (see, for example, U.S. Pat. No. 5,026,593). Another example includes using a formed strip of fiber reinforced thermoset plastic. The metal and FRPs strip are very strong yet able to be worked with in the field. However, the reinforcing material must be treated in various ways to insure an adequate bond. The aluminum must be cleaned, abraded, and chemically treated to resist oxidation before it can be used as reinforcement, or the material with fibers is abraded to “hair-up” the outermost fibers by removing the outer portions of the surrounding cured resin. In either case, the composite consists of at least an outer layer of substrate material, a coating of adhesive, a strip of reinforcing material, a second coating of adhesive, and the second outer layer of substrate. In many cases, failure does not occur from the rupturing of the reinforcement, but rather from a sheer failure along the adhesive plane bonding the elements together. In the case of the aluminum, at failure the metal “pops” free from the adhesive. This is due to the lack of surface area for even a treated strip. The fiber-based reinforcement, although having better surface characteristics, is still limited by the adhesive bond. This is due in part to the inferior bond of a resin (adhesive) to resin (fiber reinforced plastic matrix) connection. The overall bond strength depends on the adhesive coating penetrating the “haired-up” fibers that comprise only a portion of the surface area. In effect, the weak link joining the composite can be considered a “resin matrix discontinuity”.